When I moved back to my main PC and tried to import photos (after three months on the road working exclusively from my laptop and another desktop), my metadata presets were missing.
Here’s what probably happened: I had “store presets with catalog” checked on my desktop, but not on my laptop or the new desktop. So when the presets were missing on the new install, I created them anew (on the laptop) or copied them from the %appdata%\adobe\lightroom\metadata presets location. Then, during the trip, I of course saved the catalog file multiple times (every time I imported a photo and made a change). So the presets were removed from the catalog.
So when I came home and fired up lightroom on my desktop, the presets were missing. I had to go to edit –> preferences –> presets and uncheck “store presets with catalog” and then copy the presets from my laptop. My original presets are lost (though I can recover them from a photo, presumably).